How do you convert an analog signal to digital which is higher than what a ADC on a microcontroller can handle?
Posted on Oct 31, 2008 under Digital news and reports |Im trying to figure out how to get the binary value of an analog signal when the range is higher than what the converter can handle. For instance most PIC microcontroller’s come with an ADC for around 10 bits. Is there a way to still use this microcontroller with a signal of greater than 10 bits????
October 31st, 2008 at 8:20 am
Put in an attenuator on the analog side.
October 31st, 2008 at 8:20 am
10 bits is not a signal level, it’s just the resolution of the digital signal.
If your signal is higher than the ADC can handle, just attenuate the signal via a resistive divider or via an opamp. It depends on what your signal source is.
some actual numbers would help.
.
October 31st, 2008 at 8:20 am
First you have to get analog signal to be within the analog voltage range of the ADC. This is done by using an AGC or some kind of attenuation.
Assuming that your ADC analog input has a range from 0 Volt to 5 Volt. Then the ADC will convert 0 Volt to 000H and 5 Volt to 3FFH. This is un-signed conversion. If your analog signal is beyond that voltage range, it clip at 000H or 3FFH, i.e. signals above 5 Volts will be converted 3FFH. Thus AGC or attenuator is needed. Once your analog input signal is within the range of the ADC input voltage, then you are dealing with the accuracy of the conversion. 10 bits will give you 60 dB of dynamic range, which is a lot of accuracy. If you need more accuracy and your input analog signal swings from 0 Volts to 5 Volts a lot of the time, i.e. the signal has dynamic range contents of 60 dB and the spectrum of that signal is also at or near 1/2 the sampling frequency, then you are out of luck. You will need a different ADC to get more accuracy. However, if your analog signal mostly stay at a top half or lower half, i.e. it is confined to 0 Volts to 2.5 Volts or 2.5-5 Volts, then you can use an AGC or amp to scale that signal by doubling it. You can switch between 0-2.5 Volts input circuit to the ADC or a 2.5-5 Volt, i.e. you make a circuit to amplify the 0-2.5 V to be 0-5 V and another to convert 2.5-5 V to be 0-5 V. What you have now done is to basically add another bit to the convert, i.e. you’ve made it an equiv of 11 bit converter. Your software has to control the front end amp switching based on went signal is railed to either 000H or 3FFH. This only works because your signal is confined to a smaller voltage range and will move slowly from high range to low range.
Other ways to get more dynamic range is to use a log amp in front of the ADC, but you will need to calibrate the bit conversion as the 10 bit ADC is linear.
October 31st, 2008 at 8:20 am
You are stuck with 10 bit resolution on the chip, because of all the digital interference from the rest of the computer. If you need more resolution, just use an external ADC of whatever speed and resolution you need. A lot of them come with an SPI interface, which is convenient. 16 bit resolution is common, you can get 18 bit. Be very careful with the layout, screening, and grounding.
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